The present invention relates to a method of treatment of a high-level radioactive waste generated, for example, from reprocessing of spent nuclear fuels. In particular, it relates to a method for treating a high-level radioactive waste which comprises heating the radioactive waste at a high temperature, separating part of the elements contained in the radioactive waste by utilizing sublimation or boiling of each element in its various chemical forms during the heating step, and recovering a resultant residue as a solidified material.
The high-level radioactive waste generated from reprocessing of the spent fuels contains transuranium elements and Tc (technetium) having long half-lives; Cs (cesium) and Sr (strontium) that are noteworthy elements from the aspect of treatment, storage and disposal because they are responsible for the major proportion of heat generation; and valuable platinum group metals such as Ru(ruthenium), Rh(rhodium) and Pd(palladium). It is therefore very important to separate and recover them prior to solidification of the waste, and to utilize them as a radiation source, a heat generation member and a noble metal, from the point of view of effectively utilizing resources.
The following three methods are heretofore known as prior art techniques for recovering these elements from the high-level radioactive waste:
1) A solvent extraction method wherein the intended nuclides are separated by using a special solvent from the high-level radioactive waste generated from the reprocessing steps;
2) An ion-exchange method wherein the intended nuclides are separated by using an ion-exchange resin from the high-level radioactive waste generated from the reprocessing steps; and
3) A lead extraction method for platinum group elements wherein lead is added to glass at the time of the glass melting step of a vitrification process to thereby move platinum group elements to molten lead and separate them with the molten lead.
However, these prior art techniques described above are not free from the following disadvantages, respectively:
1) Since a new-type solvent is introduced to the reprocessing step in the additional solvent extraction method, the solvent treatment step becomes complicated and efficiency of the main solvent extraction step lowers conseqently.
2) Flammable materials are produced when the ion-exchange resin comes into contact with a nitric acid solution of the radioactive waste. Therefore, the ion-exchange method involves safety problems.
3) The lead extraction method for platinum group elements in the vitrification process can separate the platinum group elements but secondary treatment is necessary in order to extract them from lead.
Furthermore, none of these prior art methods can reduce the volume of the high-level radioactive waste at a high rate, whichever method may be employed.